Borehole mining of the type to which the present invention relates involves the creation of a slurry of rock particles containing recoverable mineral values by impacting a selected mineral formation with water or other aqueous solution delivered at high pressure through a nozzle lowered into a borehole. The slurry is elevated to the surface where the valuable mineral constitute is separated from the waste material.
Hydraulic borehole mining systems have several inherent advantages over conventional mining systems. Principal among them is the ability to permit the extraction of material selectively. Such systems can extract minerals that are too deep for economical strip mining without causing surface disturbances. Also, borehole mining is a far more acceptable method from both an environmental and safety standpoint than either conventional strip (open pit) or underground mines.
Borehole mining has not developed to its full potential for several reasons. One of the more applicable minerals for extraction by a borehole system is phosphate rock. Historically, shallow, low cost, strip mines have supplied the bulk of the United States needs from Central Florida. There was no need to develop this technology, as low cost rock was available and land values were sufficiently low to tolerate large acreages being tied up in waste disposal areas and reclaimed pits. In addition, conventional borehole mining was a high energy consumer because of the inefficiency of lift pumps and friction losses encountered in the transport of sufficient volumes of water at high pressure through restricted pipe sizes to create slurries.
The depletion of the low cost Central Florida phosphate reserves, the environmental restrictions opposing the development of new strip mines, and appreciating land values now increases the importance of borehole mining technology. The United States has abundant resources of deep phosphate rock both on and offshore that cannot be mined by conventional open pit or dredging methods, but can be extracted by borehole mining.
An initial part of the hydraulic borehole mining process involves, first, the drilling of a hole from the surface through a favorable mineral horizon or target zone. The hole is normally cased above the mineral zone to prevent shallower unconsolidated non-target zone materials from clogging the hole. A multi-conduit pipe, carrying a high pressure discharge nozzle (known as a "cutting jet" nozzle), is then lowered into the hole below the casing to the depth of the target zone, and water is directed against the borehole walls with sufficient pressure to break or wash particles loose from their matrix and create a slurry of solids and water which can be brought to the surface by pumping.
Easy mobility of the cutting jet is of great economic impact to the mining process. Known borehole mining systems normally permit the jet to be rotated at least to some extent without moving the mining tool, but usually require removal of the tool string in order to reposition the jet vertically. It is beneficial for a borehole system to be able to control the vertical movement of the cutting jet independently of the location of the rest of the mining tool. The effectiveness of the slurry forming system is improved when the cutting jet nozzle can be moved in a vertical manner, as well as rotated, independently of location of the slurry pump collection point. This independence of movement will allow the nozzle to be used to create slurry from different elevations in the formation, while maintaining the collection site for this slurry at the lowest level of the cavity. Maintaining the collection point at the lowest level will maximize the productivity, as the percent solids of the slurry will be the highest at that level with the volume remaining the same.
One known approach for providing both rotational and vertical freedom of movement of the cutting jet independently of the remainder of the system is described in Archibald U.S. Pat. No. 4,401,345. The Archibald system mounted the cutting jet on a separate pipe, bolted together in 20-foot sections and inserted from the surface through a slot in the remainder of the mining tool after the usual mining tool main pipe sections were already in place. The main pipe was connected to the usual swivel and hoist, and the separate jet pipe was connected to its own separate water swivel which could be raised and lowered relative to the main swivel by a separate hoist. Although this system worked, it was inefficient because of the slot restriction imposed on the area available for transporting the cutting jet and the attendant small inside diameter allowed for the associated moving cutting jet water supply line.
Prior art methods for lifting the slurry to the surface include jet pumps (also called "eductors") and force pumps located in the lower portion of the borehole. The jet pumps circulate a supply of high pressure fluid from the surface to downhole and back to the surface again, drawing the slurry with it by means of a venturi effect. The force pumps are volume displacement pumps located downhole and connected to receive either hydraulic or electric motive power from the surface.
The use of mechanical force pumps downhole can greatly complicate the construction and operation of the mining tool. Lines must be run to provide electricity or hydraulic fluid from the surface. Such pumps are apt to break down and necessitate frequent servicing, which requires removal, repair and reinsertion of the mining head with consequent expense and lost time.
The eductor pump has been the principal choice, historically, for slurry retrieval because of its simplicity. Such a pump is very inefficient, however, as the lifting effect is by high pressure water pumped downhole from the surface and back up again. The eductor pump uses one of the conduits of the multi-conduit pipe. The water passes through a nozzle that is directed through a restricted orifice in the lowest mining tool section of the multi-conduit pipe string. The resulting reduced pressure functions in the manner of a venturi within the orifice to provide the lifting action necessary to elevate the slurry. Large quantities of high pressure water are required for the powering of the pump, resulting in dilution (reduction in percentage of solids) of the recovered slurry which may cause subsequent processing problems at the surface and thereby reduce the overall productivity of the system.
The efficiency of the eductor pump is improved for hydraulic mining that takes place in a flooded borehole cavity environment which provides a hydrostatic head at the slurry intake port equal to the head differential between the elevation of the natural water table and the point of intake. The energy required to lift the slurry solids to the surface is then only that needed to overcome the friction losses of the slurry flow path in the pipe plus the head differential between the water table and the slurry discharge elevations.